This application claims priority of International application number PCT/DE00/00788, filed Mar. 10, 2000, which in turn claims priority to German patent application number 199 10 636.3, filed Mar. 10, 1999.
The invention relates to a length measurement system including one or more magnetic measuring rods and one or more magnetic field sensors.
The invention relates in particular to a magnetic length measurement system, such as is used for example for automatically determining positions, lengths and distances more particularly under rough surrounding conditions in machine engineering or in the automobile industry.
Known magnetic length measurement systems include a magnet arrangement and one or more magnetic field sensors which indicate a component of the magnetic field strength or the magnetic field direction. In the simplest case the magnet arrangement is formed by a simple rod magnet and a magnetic field sensor is formed by a magnetoresistive sensor whose output voltage is determined by the field direction. This is described in the article xe2x80x9cThe magnetoresistive sensorxe2x80x9d by A. Petersen in Electronic Components and Application 8 (1988) NO 4, 222-239. The sensor is guided at a certain distance from the magnet parallel to its north-south extension which is also the direction of measurement. The sensor plane extends in the direction of measurement and radially relative to the axis of the magnet.
A rod magnet with magnetization set in the longitudinal direction and whose length is not significantly greater than twice the width and thickness produces a magnetic field whose angle to the parallel of the longitudinal direction increases roughly linearly in the direction of measurement with increasing distance from the magnet centre. The magnetization in the magnetoresistive sensor lies in the plane of stratification and is set with a sufficiently high,field strength in the direction of the field. The output signal of the magnetoresistive sensor then changes proportional to the sine of the double angle. Since the sin(x) for small angles deviates only slightly from x a length range is produced in which the output voltage of the sensor is proportional to the position. In terms of accuracy this simplest arrangement has several drawbacks which are partly a result of the magnetic field distribution of the rod magnet and partly as a result of the properties of the magnetoresistive sensor.
The damaging marked temperature dependence of the output signal on magnetoresistive sensors and the restriction to small angular deviations from the parallel to the north-south direction of the magnet and thus to measuring lengths which are substantially smaller than the magnetic length has already been overcome by the arrangement indicated in patent DE 195 21 617. Here two magnetoresistive sensor bridges integrated onto one chip are used which supply both an output signal proportional to the sine and an output signal proportional to the co-sine of the double angle of the field direction. Through the formation of quotients the temperature-dependent amplitude no longer applies, and from the arcuate rod thus obtained it is possible to determine the angle without approximation over the entire length of the magnet. Thus measurement errors result here still mainly from the fact that there are deviations from the linear connection between angle and position. These deviations are however quite considerable for magnets where the lengths are long compared with the width and thickness. Furthermore the field strength over the middle part of a long magnet is only very slight so that the alignment of the magnetization in the resistance strip of the magnetoresistive sensors is no longer provided.
Therefore, for measuring large stretches measuring rods are used which consist of regions of uniform length magnetized alternately in the positive and negative longitudinal direction, as also indicated in DE 195 21 617. Determining the length is then carried out by counting the number of magnetized regions already passed from a starting position and adding the proportion of one region which results from the angular determination. However, information on the absolute position is no longer possible. After a breakdown of the measurement system it is necessary to return to the starting position in order to repeat the counting process.
In order to determine the absolute position it is also possible to undertake coding of the measuring rod wherein, however, uniformly magnetized regions of different length have to be used. Use of these regions results in the drawback already mentioned above of the small field strength close to the middle part of the region when several regions with the same code value lie next to each other. This problem can however be overcome by the arrangement described in EP 0 482 341 where twin tracks are used each with opposite magnetization whose direction always stands transversely to the direction of measurement. Unfortunately however in the said patent there is no arrangement given for producing the required twin track measuring rods, and the known magnetizing processes do not provide satisfactory results for the twin track.
Magnet arrangements comprised of several parts for determining position are proposed in the published specification DE 31 06 613. They are provided in order to achieve over quite specific short path lengths high local resolutions in position calculation. The drawback with all the different arrangements given is the extremely high dependence of the output signals on the distance between the magnet and sensor, which makes it necessary to provide very expensive guides for high-resolution position measurement and incurs high costs for adjustment and calibration.
DE 197 29 312 A1 describes an absolute magnetic length measurement system which contains a code track on a measuring rod and several sensors for scanning the magnetic fields which are present over the code track. One binary state of a code element in the code track is represented through a uniform magnetization and the other binary state is represented through a change in the direction of magnetization. This code track which is defined by binary states of one code element is scanned through several magnetoresistive sensors from whose output signals it is possible to determine the positions of the sensors relative to the code track. However several sensors are always required for this
In the Journal xe2x80x9cElektrotechnikxe2x80x9d 370, Volume 4 of Apr. 4, 1991 there is a review on the use of different materials for manufacturing permanent magnets of different magnetization.
The invention is concerned with the problem of providing a length measurement system having a magnetic measuring rod which is characterised by a simplified structure.
According to one aspect of the invention, the length measurement system includes at least one magnetic measuring rod in which a significant component of magnetization or the magnetization as a whole lies (substantially) in a plane which extends perpendicular to the direction of measurement, and of one or more magnetic field sensors wherein the angle of the component of the magnetization or of the magnetization as a whole changes relative to a freely selectable preferred direction (reference direction) in this plane along the direction of measurement; i.e. the direction of the component of magnetization is rotated along the direction of measurement, namely about an axis which runs along the direction of measurement. To each longitudinal position of the magnetic field sensor is thereby assigned a direction of the component of the magnetization within the measuring rod at this longitudinal position from which the longitudinal position of the magnetic field sensor can be determined.
In one embodiment, the entire cross-sectional surface of the measuring rod or of the plurality of measuring rods is uniformly magnetized. It points for example at the start of the measurement path perpendicularly upwards. As one advances in the direction of measurement the magnetizing direction is inclined increasingly relative to the preferred direction (perpendicular direction). Thus a value for the angle between the field direction and the preferred direction is assigned to each position. The magnetic field strength near the measuring rod is determined by the size of the cross-section and the material of the measuring rod. It is thus not dependent on the position in the direction of measurement. Thus it is possible to use measuring rods of any length and any angular change per length in the direction of measurement and to provide always the same measuring conditions for the magnetic field sensors which are used for determining the relevant field direction.
One embodiment uses an anisotropic magnetoresistive angle sensor for determining the direction of the magnetic field. On the chip face of the sensor there is a Wheatstone bridge, whose output signal is proportional to the sine of the double angle between the field direction and one edge of the chip and a bridge whose output signal is proportional to the co-sine of the double angle. The magnetoresistive angle sensor is attached close to the measuring rod so that its chip surface is perpendicular to the direction of measurement. Since the sine and cosine of the double angle are formed, a full period of the output signal is already obtained for a rotation of the magnetization direction relative to the output angle of 180xc2x0. Thus when using a measuring rod and an angle sensor an absolute position indication is also possible only for this angular range. The length over which the measuring rod is magnetized with the angular rotation of 180xc2x0 can be selected at will.
Since the angle measurement for determining the position is obviously liable to error, in the event where the measuring rod is magnetized so that the angle of the direction of magnetization increases linearly with the increasing position value, in the case of long measuring rods greater absolute length errors can also occur. Advantageously there are numerous arrangements which utilize the principle according to the invention of setting the direction of magnetization in the cross-sectional surface which is perpendicular to the direction of measurement so that the measuring length and measuring accuracy can be adapted according to each individual requirement.
In another embodiment, if the high measurement accuracy is only required over a part of the overall measurement path then the angular increase per unit of length is selected higher in this part than in the remaining measurement path. The part of the measurement path which can be determined with higher precision need therefore not be cohesive but can be divided up into several regions. In a special case the angular increase can also be undertaken stepwise with the position, in which case the regions of high measurement precision are the transitions between two steps. In another embodiment, if the higher measurement precision for the position is required over the entire length of the measuring rod then two parallel measuring rods are used. With the first the angle of the magnetization direction increases linearly with increasing position from the starting value and the rotation reaches 180xc2x0 at the end of the measuring rod. The angle of the magnetization direction of the second measuring rod likewise increases linearly with the position, but with a significantly higher rise so that the associated angle sensor, which with an angular rotation of 180xc2x0 indicates passing through a full period, runs through a number of periods over the entire length. The angle sensors of the first and second measuring rod are situated in a common plane perpendicular to the direction of measurement. From the indicator of the angle sensor of the first measuring rod it is possible to determine in which period of the second measuring rod the relevant sensor is situated. The indicator of the second sensor supplies the position within this period with a precision which is better by the total number of the periods of the second measuring rod than that of the angle sensor of the first measuring rod.
In a further embodiment of the invention two measuring rods are also used which are arranged in parallel. The angular rotation of the magnetization of the two measuring rods increases linearly with the position and in both cases runs through numerous periods. The number of periods over the total length differs however by one. The angle sensors of the first and second measuring rod are situated in a common plane perpendicular to the direction of measurement. From the difference in the angle indicators of the two sensors it is determined in which period of the first measuring rod the sensors are situated. The exact position is then produced by additionally taking into consideration the indicator of the sensor belonging to the first measuring rod.
In a further embodiment of the invention a very long measuring rod is divided in the direction of measurement into a number of regions of equal length. Each region contains the same number of sections of equal length. This number can be five for example. In each first section of each region the angle of the direction of magnetization rises linearly from the beginning with an increasing position value up to a certain critical angle. In the other four sections the angle of the direction of magnetization has within each section length a constant value which is however in each case greater than the said critical angle. The four discrete angular values in these four sections are assigned to numerical values. If for example four discrete angular values are different and are assigned to numbers 0, 1 , 2 and 3 then all numbers from 0 to 255 are represented in the four sections. Just as many regions can thus be characterised in the sequence of these numbers. If two angle sensors oppose each section of one region at a spacing of half the section length then it is possible to determine the special angular value in the first section and the angular values of the further four sections assigned to the numerical values in each position of the sensor arrangement relative to the measuring rod. From the determined number it is possible to indicate which region of the measuring rod is reached by the sensor arrangement. The position can be read off with high precision from the special angular value of each first section owing to the linear rise of the angle with increasing position. If the lengths of the sections are selected as 20 mm and in the first section the angle rises with the position from 0xc2x0 to 40xc2x0 and can be measured with a measurement error of less than one degree, then a total measurement length of 25.6 m can be measured with an accuracy of 0.5 mm restricted by the angular measurement. The resolution thereby achieved is at more than 15 bit.
The numerical values mentioned up until now are to demonstrate the advantages which are possible with the length measurement system according to the invention. They in no way represent a limit on that which can be reached. Thus it is possible throughout with the existing measurement precision for the angle, instead of assigning four numerical values to four discrete angular values also to assign ten numbers to ten discrete angular values and thus to obtain straightaway in the decimal system the indication of the number of the regions which lie between the beginning of the measuring rod and the actual position of the sensor arrangement.
Further embodiments of the invention avoid errors in the measurement of the position through faulty adjustment of the position of the angle sensors relative to the measuring rod. It is thus advantageous not only to use one angle sensor next to the measuring rod but also to attach two angle sensors on opposite sides of the measuring rod. If the sensors are situated on a line running through the centre point of the cross-section of the measuring rod and the magnetization of the cross-sectional surface is homogeneous then both sensors indicate the same error-free angle. If the connecting line does not however run through the centre point then the first sensor measures an angular value which is too big by a certain amount and the second sensor measures an angular value which is too small by the same amount. The mean value of the two measured values thus provides the exact value without the need for an expensive precision adjustment of the position of the sensors relative to the measuring rod.
In a further embodiment of the invention two magnetoresistive angle sensors are used which are arranged next to one another at a certain spacing in the direction of measurement. This certain spacing coincides with the length over which extends, at the beginning of the measuring rod, rotation of the direction of magnetization about 180xc2x0 and over which the output signal of the sensors runs through a complete period. The output signals of the two sensors thus coincide at the beginning of the measuring rod. In the measuring rod which is used the increase in the angle with the position in the direction of measurement does not run linearly but quadratically. As the position grows, the sensor set in this direction runs through an increasingly larger angular range than the other sensor. Owing to the quadratic increase in the angle this angular range increases linearly with the position, and the difference in the output signals of the two sensors is proportional to the position value. The position given is absolute when the sensor at the end of the measuring rod set in the direction of the growing position has run through no more than one complete sensor period than the other sensor. The advantage of this arrangement is that the measuring rod here can be turned any where about its longitudinal axis without affecting the measuring signals.
In the embodiments of the invention mentioned up until now the measuring rod was always magnetized homogeneously over its entire cross-section. However the magnetization of the cross-section of the measuring rod can also be multi-polar so that north and south poles alternate with each other around the periphery. In one embodiment, the magnetization pattern in the cross-sectional area at the starting end of the measuring rod is then rotated with an increasing angle as one advances in the direction of measurement. The advantage of a multi-polar magnetized measuring rod of this kind is that the rotation of the magnetization pattern as the position advances results in a rotation of the magnetic field near the measuring rod duplicated by half the pole number so that a considerably enhanced increase in the angle measured by the angle sensors per unit of length of the measuring rod is achieved. A significantly increased resolution of the measured length is thus provided.
In another embodiment of the invention, only a part of the cross-section of the measuring rod is made from a hard magnetic material. A characteristic of the invention is that the direction of magnetization is rotated as one advances in the direction of measurement. With circular cross-section for example the concentric inner circular part can be made from hard magnetic material and the circular ring-shaped part surrounding same can be made from a non-magnetic mechanically stable material. The mechanically stable material prevents mechanical torsion of the measuring rod about its longitudinal axis and thus helps to reduce measurement errors. The distribution of the materials in the cross-sectional surface can also be changed round so that there is a non-magnetic core and a permanent magnetic shell. This would have the advantage that the angle sensors would be attached closer to the permanent magnetic part and higher field strengths act on the angle sensors.
If the non hard magnetic core of the cross-section is made from a soft magnetic mechanically stable material it helps at the same time to eliminate errors through mechanical stress and to increse the magnetic field strength in the vicinity of the measuring rod.
In another embodiment, the inner non permanent magnetic part of the cross-section of the measuring rod is not circular but has a shape which makes it particularly resistant to mechanical deformation. It is thus also unnecessary for the permanent magnetic part of the cross section to surround completely or approximately the mechanically stable part.
A particular advantage when using the length measurement system according to the invention is that after the failure of the electric supply there is no need to undertake any standardization of the measurement system since a position is assigned to each magnetization angle. Obviously the length measurement system can also be used for purposes where the aim is not immediately to determine a length or a position but is to be used to obtain a different value, for example the weight of a seat occupant. This information can be utilised to control airbags. Thus the measurement system is particularly suitable for use in conjunction with the adjusting mechanism of an adjusting device for a motor vehicle.
Naturally dynamic parameters such as speed and acceleration can also be determined with the length measurement system according to the invention analogous with the known impulse counting measurement principles (e.g. with the combination of a rotating ring magnet and Hall element). For this it is only necessary to take into account the path stretches which can be reproduced between the different magnetization angles with the corresponding adjustment time.
Furthermore the new length measurement system is also suitable for carrying out a so-called inherent diagnosis, since a clear determination of the position of an adjusting part is possible after a breakdown. As a result of such an inherent diagnosis an adjusting system (for example an electrically operated window lifter or vehicle lock) could be moved for example from normal operation to emergency operation in order to minimize the risks for the user. Thus for example the window pane could be stopped from automatically rising up in order to reduce the risk of jamming, or for a vehicle lock the function xe2x80x9csavexe2x80x9d is cancelled to prevent a passenger from becoming locked in.